Chronic pain is a major contributor to disability in the industrialized world and is the cause of an untold amount of suffering. The successful treatment of severe and chronic pain is a primary goal of the physician with opioid analgesics being the current drugs of choice. Unfortunately, this class of compounds produces several undesirable side effects including respiratory depression, constipation, and the development of tolerance and dependence.
Opioids are derived from the opium poppy papaya somniferum and include drugs such as morphine, codeine and semi-synthetic compounds derived from them and from thebaine, another component of the opium poppy. It was hypothesized that the opioids derived their therapeutic effect by interacting with specific receptor sites in the body. Later experiments led to the belief that there were more than one receptor site in the body, in explanation for the fact that the synthetic compound nalorphine provides analgesic activity while at the same time, antagonizes the analgesic effect of morphine.
Until recently, there was evidence of three major classes of opioid receptors in the central nervous system (CNS), with each class having subtype receptors. These receptor classes were designated as μ, δ and k. As opiates had a high affinity to these receptors while not being endogenous to the body, research followed in order to identify and isolate the endogenous ligands to these receptors. These ligands were identified as enkephalins, endorphins and dynorphins.
Recent experimentation has led to the identification of a cDNA encoding an opioid receptor-like (ORL1) receptor with a high degree of homology to the known receptor classes. This newly discovered receptor was classified as an opioid receptor based only on structural grounds, as the receptor did not exhibit pharmacological homology. It was initially demonstrated that non-selective ligands having a high affinity for μ, δ and k receptors had low affinity for the ORL1. This characteristic, along with the fact that an endogenous ligand had not yet been discovered, led to the term “orphan receptor”.
The science relating to opioid receptors presents an opportunity in drug discovery for novel compounds which can be administered for pain management or other syndromes modulated by these receptor. Such drug discovery could lead to compounds having a higher affinity for the μ, δ and k receptors than known compounds, while producing less side effects.